Welcome to this tutorial about neurotransmitters. We continue to talk about our core concept, about how neurons communicate through both electrical and chemical signals, and Last time, we spent our tutorial talking about the nature of the chemical signals, the neurotransmitters. Now we're going to talk about the receptors that they interact with. So, we have some learning objectives today. I want you to be able to discuss the means by which ligand-gated ion channels affect the membrane potential of postsynaptic. Neurons. I want you to be able to compare and contrast the structure and function of the ligaand-gated ion channels and metabotropic receptors also known as the G-protein coupled receptors. We're going to focus on one very particular ligand-gated ion channel that's very important, especially in the context of learning about neural plasticity. And I want you to discuss the properties of the NMDA receptor for glutamate and why it's so important for synaptic plasticity. And lastly, I want you to be able to once again recount for the factors that determine the effect of a neurotransmitter on a postsynaptic neuron. OK. Well, let's get started again by reviewing the basic structure and functional properties of a chemical synapse and so those are recounted for you in figure 5.3, a very important figure from your textbook and what we see here is what happens when an action potentional invades the pre-synaptic terminal membrane. So there's a wave of depolarization That causes voltage gated calcium channels to open, calcium rushes in, to the pre-synaptic terminal and interacts with[UNKNOWN] that causes the snare complex, to pull together, the membranes of the synaptic vessicle and the pre-synaptic terminal, that generates a fusion pore. And neurotransmitter then can diffuse out of the synaptic vesicle where it can interact with neurotransmitter receptors on the postsynaptic surface. That's what we're going to talk about mainly in this tutorial. And so what we want to understand then are what are the consequences for activation of a receptor for a neurotransmitter on a post-synaptic cell? Alright, so that's the challenge for the day. And I'll just remind you again that we have two classes of neurotransmitters, our small molecule neurotransmitters and then our peptide neurotransmitters. And these classes of neurotransmitters interact with two broad classes Of receptors. So we will talk in length about each class We will begin with talking about our ionotropic receptors which are receptors for neurotransmitters that are part of an ion channel complex. Hence we call these ligon gated ion channels And there are metabotropic receptors, which are receptors for neurotransmitters that are not directly part of ion channels. But rather these receptors trigger a set of metabolic activities, that then might ultimately result in the opening or closing of separate ion channel molecules, as well as a variety of other post synaptic activities. Our small moleule neurotransmitters typically interact with both ionotropic and metabotropic receptors although that may not be universally true of all small molecule transmitters. Certainly our major important neurotransmitters like glutamate and GABA interact with receptors from both categories. Whereas our peptide neurotransmitters. Interact with metabotropic receptors. Okay, so let's begin to consider what exactly these receptors are and consider their structure and functional properties. So let's begin by considering the ligand-gated ion channels. So, as I said the essential Relationship here to understand is that the actual receptor sites themselves that interact with the neuro-transmitters, are part of an ion channel complex. Okay? So the receptor is part of the same molecular structure that forms the ion channel. And because of this, these are the receptor interactions that mediate the most rapid effects on postsynaptic currents. So as we go, I'll remind you that in order to understand the effect of any given neurotransmitter substance, whatever it may be. What we really need to know is something about the receptor to which that neurotransmitter bites. And we need to know something about what other receptors might also be present in the same post synaptic cell in order to understand the physiology of that transmitter molecule. So we'll see examples of that as we go along here in this tutorial. Alright, so let's consider the general molecular structure of ligand-gated ion channels, and we'll use the ligand-gated ion channel that we know best as an example. That would be the nicitonic acetylcholine receptor. So this is the receptor for the endogenous neurotransmitter acetylcholine That is also occupied and activated by nicotine. So this structure has a characteristic set of sub units that come together to comprise this ionotropic receptor. Typically there are five sub units that come together to form the functional nicotinic acetylcholine receptor. Each sub unit has an N terminus domain to it that binds the actual neurotransmitter, acetylcholine. So, the N terminus is out here somewhere. In the extracellular space. Attached to that end terminus is some number of membranes spanning helical regions and then there is some kind of cytoplasmic extension that often is a site where the receptor sub units can interact with metabolates and other kinds of molecules. Within the cytoplasm of the cell. So that's just one sub unit, a functional channel requires the aggregation of multiple sub units, typically four or five. Here in this example we see five sub units that have aggregated two alpha sub units, a beta sub unit, a delta and a gamma sub unit. And they all differ slightly in their molecular structures. But together they aggregate to form a central pore that contains the filters that result in the selectivity of this ion channel. Now as a class inotropic receptors are not as selective as our voltage gated ion channels are. So as we'll see, multiple ions can pass through the pore of this channel. Now what makes this a ligongated ion channel is the fact that channel opening is gated by the binding of the neurotransmitter to its receptor. So this ion channel is not particularly voltage sensitive, it doesn't have a voltage sensor motif among its membrane spanning domains, rather the gating mechanism is modified by the interaction of the transmitter with the receptor. That induces a conformational change that opens The pore of the channel allowing for the passage of ions. Now there are a variety of important examples of ionotropic receptors that we'll be talking about throughout the course. I've just been talking about the nicotinic acetylcholine receptor. There's a family of receptors for glutamate that are in the ionotropic category. There's a receptor, named AMPA, for the pharmacological agent that best activates that receptor. Likewise there is a receptor called the NMDA receptor N methyl D aspartate receptor, and another one called the Kainate receptor, all named... For the exogenous ligands. Of course the endogenous ligand for all three is glutemate. There's the receptor for the principal inhibitory neural transmitter of the brain, called GABA, gamma aminobutyric acid there're actually a variety of GABA receptors. here are shown some of the sub-units that come together to form what's called the GABA A receptor channel complex and so on. Receptors for glycine, for serotonin, and then for the purinergic transmitters such as adenosine Now, what you'll notice from this chart here is that there are multiple subunits that can come together to form an ion channel and combinations of subunits can form channels that have slightly different kinetic properties with respect to channel opening and closing. or, perhaps different kinds of modulatory sites that are presented to the cytoplasmic surface. Or perhaps different affinities for the endogenous ligands. So one can have different kinds of functional properties result from the aggregation of different kinds of subunits, that come together to form The entire receptor channel complex. Let's look a bit more closely at one important ionotropic receptor. And that is the receptor for the inhibitory neuro-transmitter GABA. So, what we're looking here what we're looking at here is The typical GABA A receptor channel and it's comprised of five subunits that come together, in this case an alpha, two beta subunits, a gamma, perhaps a delta or another alpha that have come together here. And like the nicotinic acetylcholine receptor these five. Five subunits aggregate to form a pore, through which ions may pass. But what I want you to notice here, is really the complexity of this ion channel. there's a variety of binding sites for other substances that can modulate, The efficiency at which these channels open or close. for example, there is a site for the binding of steroids. For the binding of barbiturate drugs many of our pills that we take at bedtime that can modify the way people fall asleep are acting at a site deep within this receptor complex here. There are sites on the extra-cellular side that likewise can modulate the flux of current through these channels. Again drugs that are useful pharmacologically and are important drugs in the medical armamentarium and a variety of other binding sites for drugs that are not illustrated here, including alcohol. Now this particular ionotropic receptor allows for the passage of chloride ions. So chloride ions pass through a selectivity filter here which are favorable for this monovalent anion to pass down it's concentration gradient as illustrated here that concentration gradient is shown inside the cell to outside the cell. Which is typical of an immature neuron as we'll talk about in a little while. Now, let's talk about the currentst that pass through lignad-gated ion channels. And in order ot understand the principles involved we need to revisit Ohm's law. So you will recall that we talked about Ohm's law. As a being V equals IR and we, we were able to rearrange that in terms of current and indicated that current equals the inverse of resistance which is the conductance term times V. And that V term we could expand In terms of the driving force for a permeant ion as a difference between the membrane potential and the reversal potential for the permeant ion. So we can apply the very same priniciples to the movement of ions through a ligand gated ion channel What's different here, is that the channel is gated, not by voltage, but by the binding of a neurotransmitter to it's receptor. So when we do that, what we see is that the current which, here, is called PSC , for Post Synaptic Current. Is equal to the ligand-gated conductance, the g term, times the driving force. Now, if the conductance of this channel happens to be greater than zero this is what would happen when a neuro-transmitter binds. Then we would expect there to be current. Provided that we have driving force, if there is no driving force, that is if the membrane potential is precisely the reversal potential for the active conductants. Then even though we may have a significant value of conductants then there will be no post synaptic current, if the driving force term equals zero. Okay. Now, I want us to consider how this version of Ohm's law applies to understanding the conductance of the nicotinic acetylcholine receptor that we have at our end plates on our muscle fibers. So let's turn our attention to the neuromuscular junction, so this is the place where our alphamotor neurons synapse upon our skeletal muscle fiber. As illustrated over to the right. And what we see here is an axon that makes contact with a muscle fiber. And we can experiment upon this prepration. I'd like to take you through some data that are derived from such experiements. So notice that we can stimulate the axon with a micro-electrode we can voltage clamp the muscle fiber with a voltage clamp amplifier so we could measure the membrane potential, we can pass current to establish whatever membrane potential is of interest to the experimenter. And so what we're looking at here is a recording of end plate currents. So this is a, kind of a post synaptic current recorded from a muscle fiber. And notice that the muscle fiber is voltage clamped at different levels. from a rather hyperpolarized potential of 110 millivolts. All the way up through an extreme depolarized potential of plus 70 millivolts. And when we first look at the extreme hyperpolarization, what we find is that there is a significant inward current. So again negative current here by convention is an inward current carried by a cation. So with 110 millivolts, of hyperpolarization, stimulating this axon leads to a strong inward current. If we cut that hyperpolarization in roughly half, and now set the membrane potential to minus 60 millivolts We have about half as much inward current flowing into that muscle fiber. Now notice what happens when we voltage clamp to zero millivolts. We see essentially no curret at all in this muscle fiber when we stimulate the axon. So, keep that in mind. And finally, if we are to depolarize the muscle fiber to plus 70 millivolts and then stimulate the axon, what we see is not an inward current at all, but an outward current. So this behavior should look somewhat familiar to you because we saw similar behavior when we considered the impact of membrane potential. On the currents that pass through voltage gated ion channels, only here the ion channel's gated not by voltage but by the binding of[UNKNOWN] to its receptor. So not for example, that we have inward currents at hyper polarized potential, outward currents at depolarized potential And no net current at 0 millivolts. So that should, prehaps give you some insight, as to what might be the currents that are flowing through this ion channel. Now, in order to help you think through that, I want you to be able to relate our version of Ohms law, that we're revisiting here. In the context of looking at end plate currents at the neuromuscular junction, and I want you to think about the following set of study questions that you're about to see. And they're also prepared for you at the end of the tutorial notes. So why don't you take a few minutes now, and pause the recording, and consider the following set of study questions.
